The object of the present patent is to describe a method for improving the spatial resolution of displacement meters including laser confocal and triangulation displacement meters by using a cantilever to detect topographical variations, without feedback control of the cantilever position. The terms triangulation meter instead of triangulation displacement meter and confocal meter instead of confocal displacement meter may be used interchangeably. Topographical variations are measured by directly determining the change in disposition of the cantilever to provide a three dimensional topographical reconstruction. Triangulation displacement meters, or alternatively, confocal displacement meters are used to measure the vertical (out-of-plane) movement of the cantilever from the light reflected off the cantilever, and correlate it to the horizontal movement of the cantilever on the sample.
Laser confocal and triangulation displacement meters for measuring topographic variations have matured enough to allow nanometer resolution in the vertical (out-of-plane) direction. Laser confocal displacement meters have been described in literature and are commercially available. Both types of instruments find applications in a number of areas including thickness measurements, alignment, topography measurements, step height measurements, flatness measurements, profile measurements etc. However, these techniques provide inadequate spatial resolution for many applications due to the laser beam diameter, which is typically larger than 2 μm and often about 20 μm.
Scanning probe methods developed within the last two decades offer high-resolution images of sample properties. Scanning probe microscopes (SPM) measure properties at localized spots, such as: topography, thermal conductance, temperature, capacitance, optical absorption, or magnetism. They all use a cantilever with a sharp tip at a very close proximity or in contact with the sample. This close proximity allows for very high resolution. The image is formed by scanning a cantilever over the sample while measuring the desired property. Unlike light based microscopes such as laser confocal and laser triangulation displacement meters, scanning probe microscopes are not wavelength limited; hence their resolution is limited only by the size of the tip at the edge of a cantilever and not by the diffraction effects of light.
The atomic force microscope (AFM) is one of many types of SPM. AFM's employ: a cantilever, a light source, an electronic feedback circuit controlling the out-of-plane (vertical) position of the cantilever, an X-Y-Z piezoelectric transducer, and a photodetector. As the cantilever moves horizontally relative to the sample, topographical variations of the sample change the light reflected off the cantilever. A four-quadrant detector measures the reflection. A closed loop piezoelectric feedback control controls the vertical position of the cantilever. The feedback of the AFM counteracts the signal produced by the four quadrants detector. The cantilever or the sample is moved to maintain the cantilever and the light reflected from it at a constant angle. In almost all SPM's, cantilever positioning is achieved with piezoelectric transducers such as cylindrical piezotubes. Applying a voltage between electrodes of the piezotube causes the length of the tube to change with a limited maximum motion along the tube axis depending on the tube length.
A combination of confocal or triangulation displacement meters with a cantilever can operate without the need for closed loop piezoelectric feedback control. This type of arrangement would allow for improved spatial resolution of confocal or triangulation displacement meters without the additional complexity of an AFM. The cantilever can be easily added and separated from the displacement meter. Confocal and triangulation displacement meters have been used with a cantilever to produce topographical surface maps by the inventors.
Triangulation displacement systems have been widely used. Their use has also been reported in the semiconductor industry for a number of applications including: inspection, quality control, and defect detection of integrated circuits during various manufacturing stages, measuring the change in thickness of a wafer and other planarizing parameters in processes such as chemical-mechanical polishing, and inspection of chip packages.
Triangulation displacement meters measure the position of an object by tracking the light reflected from the target surface. A light beam, typically laser (including a superluminescent laser or diode), is projected on an object. Other light sources such as collimated light or room light may be used. The reflected beam is focused through a lens on a light-receiving element (photodetector) such as a position sensitive device (PSD) or charge coupled device (CCD). As the scan of the sample progresses, variations in the sample topography lead to variations in the position of the reflected signal as measured by the photodetector. A number of mathematical algorithms can be used to calculate the topography from the change in the signal on the photodetector and from the geometry of the set-up. When the term triangulation displacement meter or triangulation meter is used herein it refers to a system that uses any type of light source reflected on a photo-detector to track displacement changes.
Confocal meters have been used for a number of applications, including: surface characterization, measuring the position of micro objects, highly reflective surface measurements, MEMS devices evaluation, characterization of biological structures, and measurement of solder, gold, and stud bumps.
In a typical laser confocal displacement meter, a lens attached to a tuning fork focuses a laser beam on the surface of the sample. The tuning fork oscillates a lens rapidly in the vertical (out-of-plane) direction, focusing and defocusing the laser on the sample. The beam returned from sample is reflected by a half mirror and focused on a pinhole. A peak signal is formed on a receiving element when the focal plane coincides with the sample. A detector transforms the light signal to an electrical signal. Changes in surface reflectance do not affect the focal position and, therefore, the topography measurement.